1. Field of the Invention
The present invention relates to an image projection system implemented with a spatial light modulator (SLM). Particularly, this invention relates to an image projection system that is implemented with a SLM with a reduced size and manufactured at a lower cost.
2. Description of the Related Art
Even though there have been significant advances made in recent years in the technologies of implementing electromechanical micromirror devices as spatial light modulators (SLM), there are still limitations and difficulties when they are employed to display high quality images. Specifically, when the display images are digitally controlled, the quality of the images is adversely affected because the images are not displayed with a sufficient number of gray scale gradations.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirrors and each of these micromirrors are controlled for modulating and projecting a display pixel. Depending on the resolution requirements of the displayed images, the number of required micromirrors ranges from 60,000 to several million for each SLM.
Referring to FIG. 1A for a digital video system 1 includes a display screen 2 disclosed in a relevant U.S. Pat. No. 5,214,420. A light source 10 is used to generate light beams to project illumination for the display images on the display screen 2. The light 9 projected from the light source is further concentrated and directed toward lens 12 by way of mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate the light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. FIG. 1B shows a SLM 15 that has a surface 16 that includes an array of switchable reflective elements 17, 27, 37, and 47, each of these reflective elements is attached to a hinge 30. When the element 17 is in an ON position, a portion of the light from path 7 is reflected and redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge on the display screen 2 to form an illuminated pixel 3. When the element 17 is in an OFF position, the light is reflected away from the display screen 2 and, hence, pixel 3 is dark.
Each of the mirror elements constituting a mirror device functions as a spatial light modulator (SLM), and each mirror element comprises a mirror and electrodes. A voltage applied to the electrode(s) generates a coulomb force between the mirror and the electrode(s), making it possible to control and incline the mirror. The inclined mirror is “deflected” according to a common term used in this patent application for describing the operational condition of a mirror element.
When a mirror is deflected with a voltage applied to the electrode(s), the deflected mirror also changes the direction of the reflected light in reflecting an incident light. The direction of the reflected light is changed in accordance with the deflection angle of the mirror. The present patent application refers to the light reflected towards a projection path designated for image display as “ON light”, and refers to a light reflected in a direction away from the designated projection path for image display as “OFF light”. When the light reflected by the mirror to the projection path is of lesser intensity than the “ON light”, because only a portion of the reflected light is directed in the ON light direction, it is referred to as “intermediate light”. The present patent application defines an angle of rotation along a clockwise (CW) direction as a positive (+) angle and that of a counterclockwise (CCW) direction as a negative (−) angle. A deflection angle is defined as zero degrees (0°) when the mirror is in the initial state.
The on-and-off states of the micromirror control scheme as that implemented in the U.S. Pat. No. 5,214,420, and in most conventional display systems, impose a limitation on the quality of the display. Specifically, applying the conventional configuration of a control circuit limits the gray scale gradations produced in a conventional system (PWM between ON and OFF states) limited by the LSB (least significant bit, or the least pulse width). Due to the ON-OFF states implemented in the conventional systems, there is no way of providing a shorter pulse width than the duration represented by the LSB. The least intensity of light, which determines the gray scale, is the light reflected during the least pulse width. The limited levels of the gray scale lead to a degradation of the display image.
Specifically, FIG. 1C exemplifies, as related disclosures, a circuit diagram for controlling a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a based on a Static Random Access switch Memory (SRAM) design. All access transistors M9 on an Row line receive a DATA signal from a different Bit-line 31a. The particular memory cell 32 is accessed for writing a bit to the cell by turning on the appropriate row select transistor M9, using the ROW signal functioning as a Word-line. Latch 32a consists of two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states, including a state 1 when Node A is high and Node B is low and a state 2 when Node A is low and Node B is high.
FIG. 1A shows the operations of the switching between the dual states, as illustrated by the control circuit, to position the micromirrors in an ON or an OFF angular orientation. The brightness, i.e., the gray scales of a digitally controlled image system is determined by the length of time the micromirror stays in an ON position. The length of time a micromirror is in an ON position is controlled by a multiple bit word.
If each pixel is equipped with SRAM and is configured to perform the ON/OFF control for the pixel in synch with the bias driving of the micromirror as described for FIG. 1C above, the following technical problem is anticipated.
That is, the characteristics of the MOS transistor and semiconductor memory can possibly be varied by being irradiated with light. Therefore, if an incident light to be modulated enters the circuit structure, which is equipped inside the substrate and which is used for controlling a mirror, in a spatial light modulator operating while receiving relatively intense light, a malfunction will be caused.
Accordingly, the reference patent documents listed and numbered below, Patents 4, 5, 6, 7 and 8 have disclosed countermeasures to prevent light from being irradiated onto the substrate of a spatial light modulator. However, these techniques, which all involve adding some kind of structure to the inside of the substrate, are faced with a technical problem that the internal structure of the spatial light modulator becomes more complex.    [Patent document 1]: U.S. Pat. No. 5,214,420    [Patent document 2]: U.S. Pat. No. 5,285,407    [Patent document 3]: U.S. Pat. No. 5,083,857    [Patent document 4]: U.S. Pat. No. 5,818,095    [Patent document 5]: U.S. Pat. No. 7,206,110    [Patent document 6]: U.S. Pat. No. 7,230,749    [Patent document 7]: U.S. Pat. No. 7,304,337    [Patent document 8]: US Patent Application 20070206269